1. Field of the Invention
The present invention relates to a power supply for a motor, and more particularly, to a motor power supply device which reduces a redundant usage of components and provides a satisfactory value of resistance for motor speed of a brake circuit by sharing a dynamic braking circuit and an inverter. Whereby, size of products and manufacturing expense thereof can be reduced.
2. Description of the Related Art
A three-phase motor has a coil wired in a triangle shape and a power supply device for the three-phase motor generates a three-phase voltage to operate the motor. As shown in FIG. 1, the three-phase motor power supply comprises a AC (Alternating Current) power supply part 101 supplying 110V/220V commercial AC power; a rectifier 103 rectifying the AC power generated from the AC power supply part 101 to a DC power; a capacitor 115 smoothing out the rectified voltage in the rectifier 103; an inverter 116 inverting the DC power from the capacitor 115 to a AC power having various frequencies and generating a three-phase voltage. The inverter 116 has a plural number of transistors turned on/off depending on a PWM (Pulse Width Modulation) control signal, and diodes connected in parallel with each of the plural number of transistors. The three-phase motor power supply further comprises a microprocessor (not shown) turning on/off the transistors of the inverter 116, responding to the PWM control signal and modulating power frequencies to control a rotation speed of an AC motor 117.
A motor power supply device generally comprises an inrush-current protection circuit formed to consume inrush-current flowing from an inrush-current resistor 102 to the capacitor 115 by turning off a relay 111 for inrush-current protection when an initial power is applied; an over voltage protection circuit 130, connected across the inverter 116, protecting the capacitor 115 from an over-voltage condition and including an over voltage protection resistor 112 and a diode 113 connected, in parallel, in series with a control switching element 114; and a dynamic braking circuit shortening power input terminals of the AC motor 117.
The dynamic braking circuit has dynamic braking resistors 120 connected with a braking relay 122 across each pair of power input terminals U and V; U and W of the AC motor 117.
As shown in FIG. 2, the three-phase motor power supply comprises a AC (Alternating Current) power supply part 201 supplying 110V/220V commercial AC power; a rectifier 203 rectifying the AC power generated from the AC power supply part 201 to a DC power; a capacitor 215 smoothing out the rectified voltage in the rectifier 203; an inverter 216 inverting the DC power from the capacitor 215 to a AC power having various frequencies and generating a three-phase voltage. The inverter 216 has a plural number of transistors 254b turned on/off depending on a PWM (Pulse Width Modulation) control signal, and diodes connected in parallel with each of the plural number of transistors 254b. The three-phase motor power supply further comprises a microprocessor (not shown) turning on/off the transistors 254b of the inverter 216, responding to the PWM control signal and modulating power frequencies to control a rotation speed of an AC motor 217.
A motor power supply device generally comprises an inrush-current protection circuit formed to consume inrush-current flowing from an inrush-current resistor 202 to the capacitor 215 by turning off a relay 211 for inrush-current protection when an initial power is applied; and a dynamic braking circuit 230 to brake the AC motor 217.
The dynamic braking circuit, as shown in FIG. 2, having pairs of dynamic braking diodes 224, respectively, connected to each of the power input terminals of the AC motor 217. Further the respective dynamic braking diodes 224 may be parallel connected to a dynamic braking resistor 220 and a braking relay 222.
Further, as shown in FIG. 1 or 2, the braking relay 122 or 222 stays in an off state when the AC motor 117 or 217 is in a driving mode, and the braking relay 122 or 222 is turned on when the motor 117 or 217 is about to stop or after the AC motor 117 or 217 is stopped. When the braking relay 122 or 222 is activated, the AC motor 117 or 217 stops suddenly or prevents the AC motor 117 or 217 from rotating due to an external force.
However, the dynamic braking circuit only operates while the AC motor 117 or 217 is stopping or after the motor 117 or 217 is stopped in the conventional dynamic braking circuit. The dynamic braking circuit does not operate while the AC motor 117 or 217 is regularly in the driving mode when power is supplied and the capacitor 115 or 215 is charged with the power. However, the dynamic braking circuit is independently provided with components to perform an operation as required, resulting in an increase in a size of an entire circuit and an increase in an expense for a manufacture thereof.
A size of the dynamic braking resistor 120 or 220 depends on amounts of current flowing through the shortcircuited AC motor 117 or 217, and the amounts of the currents therein depend on a preceding speed when the AC motor 117 or 217 is about to stop. Thus, a resistance value of the dynamic braking resistor 120 or 220 is increased as the AC motor 117 or 217 rotating at high speed tries to stop. The resistance value of the dynamic braking resistor 120 or 220 is decreased as the motor rotating at slow speed tries to stop. Therefore, a motor speed may determine the resistance value of the dynamic braking resistor 120 or 220. However, the conventional AC motor 117 or 217 was not designed in consideration of such a configuration.
Thus, the present invention provides a motor power supply in which a number of elements of a dynamic braking circuit and an inverter share a diode and also reduce a size of products and a manufacturing expense thereof. Further, with an effect that a value of resistance in a braking mode is variable depending on a motor speed, a satisfactory value of resistance for a braking circuit is provided.